The historic type area of the Dakota Formation lies along the Missouri and Big Sioux River Valleys in northeast Nebraska and northwest Iowa, in the eastern margin area of the Western Interior Province. The type Dakota sequence is characterized to provide a basis of comparison with supposed “Dakota” sections elsewhere in the Western Interior. The general sequence includes (1) a lower sandstone-dominated Nishnabotna Member with coarse-grained and conglomeratic facies, and (2) an upper mudstone-dominated Woodbury Member with sandstone channel bodies and lignites. The depositional sequence is interpreted to record, in ascending order: (1) aggradation of coarse-grained braided and proximal meanderbelt fluvial systems; (2) an episode of lowered base levels, channel incision, and widespread pedogenic alteration; (3) aggradation of channels and flood-basin sediments in meander-belt fluvial systems; and (4) upward transition into deltaic and marginal-marine facies. The Woodbury Member is of Cenomanian age, and is interpreted to have aggraded in response to sea-level rise in the adjacent seaway during transgressive phases of the Greenhorn Cyclothem, with progressive eastward displacement of nonmarine by marine facies. The age of the lower Dakota sequence is known with less certainty, but is probably upper Albian in part. Aggradation of lower Nishnabotna fluvial systems may be related to sea-level rise during transgression of the Kiowa–Skull Creek Cyclothem, although the possibility that the entire Dakota sequence is post-Kiowa cannot be completely discounted. Based on the type sequence, usage of the term Dakota Formation should be restricted to include only eastern-derived detrital sequences and should exclude marginal-marine or marine facies of the Kiowa–Skull Creek Cyclothem. Usage of the term “Dakota” for western-derived sedimentary sequences in the Rocky Mountain area is discouraged.

The mid-Cretaceous (Albian-Cenomanian) boundary in the central United States traditionally is placed at a major unconformity that records a withdrawal of marine waters from the Western Interior Seaway. Although this unconformity can be located with reliability in many areas of western North America and the Gulf Coast, its placement along the eastern margin of the interior seaway is more problematical. Largely as a matter of convenience, unconstrained by biostratigraphic data, the base of Cenomanian strata has been placed at the base of the sandstones of the Dakota Formation in its type area of eastern Nebraska and western Iowa, where they overlie Paleozoic and Precambrian rocks. Palynological evidence from Dakota sections in the type area now suggests that the mid-Cretaceous unconformity may instead be traceable to a horizon within the formation, separating the upper Woodbury Member from the lower Nishnabotna Member. If this interpretation proves to be correct, the Nishnabotna is chronostratigraphically and depositionally equivalent to Albian units such as the Newcastle and Muddy Sandstones of Wyoming and the subsurface “J” Sandstone of the Denver Basin. Given the historical development of the use of the term “Dakota,” this suggested interpretation also calls into question the advisability of continuing to include the Nishnabotna Member within the Dakota Formation.